John S Inscribed thus:

On 11/7/2011 8:51 PM, J. C. Mc Laughlin wrote:
Dear John S: Conventional wisdom and common sense suggests that
measuring Zin (with an open and then a short at the far end) at a
frequency where the transmission line looks like an odd multiple of
1/8 WL tends to provide the best quality of measurements to be used
to characterize a piece of coax. Such measurements tend to result in
two numbers that are similar. Extrapolation to 434 MHz should provide
reasonable estimates.

The UHF version of the AIM4170 and its software will provide the
values and do the indicated calculations. Of course, one needs to
select the reasonable value (from the infinite inherently provided)
for rad/m - but that is rarely an issue. Your equipment too should be
able to provide the two values of Zin and a good HP calculator will
do the rest.

Measurements near, say, frequencies where the coax looks like
multiples of 1/4 WL produce numbers that are not favorable for
calculation. Baron provides other ways to think of the task.

No doubt you know this, but others might not. 73, Mac N8TT

"Baron" wrote in message ...

John S Inscribed thus:

I have about 94 inches of RG-142B/U. I am using a Fluke 6061A signal
generator, an HP 8405A Vector Voltmeter, and a Narda dual
directional coupler. I have tried to measure the line
characteristics at 434 MHz but I am not satisfied that the results
are accurate. It is very difficult to get good short and open
circuits at this frequency and I also wonder if the 8405A accuracy
suffers since a short is well away from the nominal system impedance
of 50 ohms.

What if I simply calibrate the 8405 with a short on the end of the
line (the measurement plane) then attach my antenna and accept the
readings? Will they be very far from the real value?

Thanks,
John KD5YI

The easiest way to get the characteristics of the line is to look up
the manufacturers data. Somehow I don't think that this is really
what you are looking for !

Irrespective of line length if its terminated in its characteristic
impedance then you will only measure unity vswr. Open or short
circuit terminations are easy enough to obtain. Having a known input
quantity and measuring the return value will give you the line loss
for that particular line length.

I suspect that its actually the antenna characteristics that you are
seeking to measure ! In which case I would use a line, accurately
cut, to be number of half waves long, then the impedance presented at
the far end would be repeated at the near end. Of course you would
need to have an accurately cut quarter wave length in order to
determine whether the load was inductive or capacitive in nature.

I'm sure that if I'm mistaken some of the more knowledgeable will
correct my errors.

HTH

Yeah, well, sometimes I get turned around in my quests and lose my
way.

It is the antenna characteristics I am after. What I want to know is,
do I need to know the transmission line characteristics which I use
during the test in order to modify my test results to show the true
antenna impedance?

What I want to do is build an antenna based on its radiation
characteristics (as shown with EZNEC) and then measure its impedance
(at the end of a few inches of parallel conductors) so that I can put
in a matching network to give my source what it wants.

John

Surely the antenna will have some means of adjustment ie gamma match,
and since you know that if the electrical length of cable equals a
number of half waves, adjusting the antenna to show minimum VSWR at
generator end, achieves your goal...

--
Best Regards:
Baron.